This invention relates to a method of synthesizing cubic crystal structure boron nitride, and more particularly, to a method of synthesizing cubic crystal structure boron nitride by subjecting a mixture of a hexagonal boron nitride or other boron compound and a catalyst to a high temperature and a high pressure, which method is characterized by the use, as the catalyst, of silicon, silicon nitride, a silicon alloy mixture or nitride thereof or a silicon-and-aluminum containing mixture.
In a prior method of synthesizing cubic boron nitride, which is described for example in U.S. Pat. No. 2,947,617, hexagonal boron nitride and a catalyst are exposed to high pressures, above 40,000 atmospheres, and elevated temperature, above 1,200.degree.C. In this process catalysts are employed which include alkali metals, alkaline-earth metals, antimony, tin, and lead. Certain of these catalysts are unstable and may be easily oxidized, however, and it is not easy, therefore, to control the process.
Another method for synthesizing cubic boron nitride employs lower pressures and lower temperatures and alloys of silver and cadmium as catalyst.
In this latter method, the cubic boron nitride may be synthesized at lower pressures of about 30,000 atmospheres and at lower temperatures, under 1,000.degree.C, but the conversion ratio from hexagonal to cubic boron nitride is so low that the maximum yield is only 20%. Accordingly this method is not economical for the production of cubic boron nitride. It is, therefore, an object of the present invention to provide a method which will avoid the above mentioned problems and efficiently produce cubic boron nitride employing a catalyst which is stable and easy to handle.